I agree with Chris about the warm up time with the BRB sleeves. That material seems to take a while to soak up the heat (for lack of better termonology) than brass. I always to try to wait to tune the BRB's until they have a chance to warm up. (I always let the engine warm up before putting it on the track) I think this is one of the advantages to that material. Since it takes a while to heat up, it's probably less resistant to deformation keeping a better cylinder for the piston. Less expansion also means being able to handle higher temps without issue even if it's not the best performance, just gets a little soft is all. That said I'd also like to point out that the BRB's need a higher operating temp as well. They want to run over 210F to loosen up a bit.